JPH05153518A - Aft circuit - Google Patents

Abstract

PURPOSE:To attain the non-adjustment of the frequency and to miniaturize a circuit by constituting a frequency discrimination resonance circuit with an end face reflection type surface wave resonator using a BGS wave. CONSTITUTION:The resonance circuit 19 is connected to an AFT detector 8 to provide a desired frequency discrimination characteristic to the detector 8. The circuit 19 has the structure for connecting a resistor 21 and a capacitor 22 in parallel with a surface wave resonator 20. An interdigital transducer composed of comb-shaked electrodes 24, 25 and 25, 26 is directly connected onto a piezoelectric substrate 23 in the resonator 20. The resistor 21 is a damping resistor adjusting the AFT sensitivity and the resistance is properly selected in the range of 100ohm-100kohm. The capacitor 22 is used to cut off a DC component. Thus, a prescribed frequency discrimination characteristic is imparted to a detector 8 without frequency adjustment and the AFT circuit is miniaturized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AFT circuit for a television receiver, and more particularly to an AFT circuit having an improved circuit for supplying a reference frequency to an AFT detection circuit.

[0002]

2. Description of the Related Art A conventional AFT circuit will be described with reference to FIG. In the television receiver, the signal received by the antenna 1 is given to the tuner 2. The tuner 2 has a high frequency amplifier 3, a mixer 4 and a local oscillation circuit 5. The received signal is amplified by the high frequency amplifier 3 and converted into a video intermediate frequency (IF) signal by the mixer 4 and the local oscillation circuit 5. In the latter part of the tuner,
An intermediate frequency amplifier 6 is connected, the IF signal is amplified in the intermediate frequency amplifier 6, is given to a video detector 7, and the video signal is detected by the video detector 7 and output.

By the way, in the above television receiver,
It is important that the IF signal be kept at a predetermined reference frequency. For example, in a color television receiver, if the local oscillation frequency given by the local oscillation circuit 5 shifts,
The frequency of the IF signal is shifted, and as a result, no color appears at all, insufficient contrast occurs, or the color becomes too dark. It also causes troubles such as cross color, beat fringes, smear or ringing. Therefore, in order to keep the IF signal at a predetermined reference frequency, an AFT circuit is conventionally configured between the intermediate frequency amplifier 6 and the local oscillation circuit 5. AFT circuit is A
It includes an FT detector 8 and a resonance circuit 9. AFT detector 8
Has a frequency discriminator having the characteristics shown in FIG.

In the AFT detector 8, the intermediate frequency amplifier 6
Is detected by the frequency discrimination characteristic provided by the resonance circuit 9, and the detection output voltage (error voltage) based on the deviation from the reference frequency f ₀ is given to the local oscillation circuit 5 as the AFT control voltage to cause local oscillation. The circuit 5 operates so that the oscillation frequency of the circuit 5 is controlled to be constant. The reference frequency f ₀ varies depending on the country of use,
For example, 45.75 MHz, 58.75 MHz, 38.
9 MHz or the like.

[0005]

DISCLOSURE OF THE INVENTION The conventional AFT described above
In the circuit, the resonance circuit 9 that gives the frequency discrimination characteristic as shown in FIG. 3 is composed of an LC resonance circuit in which an inductance and a capacitance are connected in series and parallel. Therefore, AF
In the T circuit, adjustment of the inductance L is indispensable, and no adjustment is required. If a ceramic resonator is used, the center frequency of the AFT circuit can be made unadjusted, but the above-mentioned frequency band, that is, 45.75 MHz,
In the frequency band such as 58.75 Hz or 38.9 MHz, there was no suitable ceramic resonator. For example, a bulk ceramic resonator has a resonance frequency of ˜30 MHz, and a surface acoustic wave resonator has a resonance frequency of 60 MHz or higher. Further, even if a resonator capable of resonating in the frequency band as described above can be constructed by using a ceramic resonator, the capacitance becomes very large and the element shape becomes large, and the resonance of the AFT circuit described above occurs. It could not be used in circuit 9.

An object of the present invention is to provide an AFT circuit which is equipped with a resonance circuit capable of making frequency adjustment unadjusted and which can be easily downsized.

[0007]

SUMMARY OF THE INVENTION The present invention provides a frequency discrimination characteristic in an AFT circuit that detects a video carrier of a television receiver and applies a detection output voltage to a local oscillation circuit based on a deviation from a reference frequency. The resonance circuit to be provided is such that a plurality of interdigital transducers each including a pair of comb-teeth electrodes having a plurality of electrode fingers to be interleaved are connected in series on the piezoelectric substrate, and the BGS is provided.
An edge reflection type surface wave resonator utilizing waves and a resistance element connected in parallel with the surface wave resonator are provided.

[0008]

[Function] The resonance circuit for frequency discrimination in the AFT circuit is BGS
Since it is composed of an end-face reflection type surface wave resonator using waves, it is possible to eliminate frequency adjustment. Further, in the above-mentioned surface acoustic wave resonator, since a plurality of interdigital transducers are connected in series, the capacitance is also reduced.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be clarified by describing embodiments of the present invention with reference to the drawings. Figure 1
FIG. 3 is a schematic configuration diagram of a tuner portion of a television receiver including an AFT circuit according to an embodiment of the present invention. The tuner 2 is connected to the antenna 1, and the input signal received by the antenna 1 is given to the tuner 2. The tuner 2 includes a high frequency amplifier 3, a mixer 4 and a local oscillator circuit 5.
Have. The tuner 2 has the same structure as the tuner 2 of the conventional television receiver shown in FIG.

The intermediate frequency signal given from the mixer 4 of the tuner 2 is given to the intermediate frequency amplifier 6, amplified by the intermediate frequency amplifier 6, given to the video detector 7, and the video signal is given by the video detector 7. It is detected and output. An AFT circuit is connected to the intermediate frequency amplifier 6, and the AFT circuit has an AFT detector 8 and a resonance circuit 19. The intermediate frequency amplifier 6 and the image detector 7
The respective configurations of the AFT detector 8 and the AFT detector 8 are similar to those of a conventionally known television receiver.

The feature of this embodiment resides in the resonance circuit 19 for giving the AFT detector 8 a desired frequency discrimination characteristic.
That is, the resonance circuit 19 includes the surface wave resonator 20, and the resistor 21 and the capacitor 22 connected in parallel to the surface wave resonator 20. With the conventional surface acoustic wave resonator, it is difficult to obtain a resonator that resonates at the reference frequency f ₀ of the television receiver, and even if it is obtained, the capacitance becomes too large to be used. On the contrary,
In this embodiment, the surface acoustic wave resonator 20 utilizing the BGS wave shown in FIG. 4 is used. Referring to FIG. 4, surface acoustic wave resonator 20 is configured using a rectangular piezoelectric substrate 23 that is polarized in the direction of arrow P. The piezoelectric substrate 23 is made of, for example, a piezoelectric ceramic material such as lead zirconate titanate-based piezoelectric ceramic.

On the upper surface of the piezoelectric substrate 23, a first comb-teeth electrode 24 having a plurality of electrode fingers 24a, 24a, a second comb-teeth electrode 25 having a plurality of comb-teeth electrodes 25a, 25a, 25a, and A third comb tooth electrode 26 having a plurality of comb tooth electrodes 26a, 26a is formed. The plurality of comb-teeth electrodes 24a and the plurality of comb-teeth electrodes 25a are arranged so as to be interleaved with each other, while the plurality of comb-teeth electrodes 26a are also interleaved with the plurality of comb-teeth electrodes 25a. It is arranged to fit. Therefore, on the upper surface of the piezoelectric substrate 23, the first interdigital transducer composed of the comb-teeth electrode 24 and the comb-teeth electrode 25 and the second interdigital transducer composed of the comb-teeth electrode 25 and the comb-teeth electrode 26 are connected in series. A connected structure is formed.

As shown in the plan view of FIG. 5, the distance between the facing inner edges of the adjacent electrode fingers of the comb-teeth electrodes 24 to 26 is defined as the wavelength of the excited BGS wave being λ. When they did, they were all set to λ / 4. In addition, the piezoelectric substrate 2
The width of the electrode fingers formed in contact with both end surfaces of 3 is λ / 8, and the widths of the other electrode fingers are λ / 4. In the surface acoustic wave resonator 20, by applying an AC voltage between the comb-teeth electrodes 24 and 25, the BGS wave is generated by the electrode fingers 24a to 24a.
It is excited in a direction orthogonal to 26a and in a direction opposite to the orthogonal direction, is reflected between the end faces 23a and 23b of the piezoelectric substrate 23, and operates as a resonator.

In the surface wave resonator 20 using the BGS wave as described above, a surface wave resonator having a resonance point in the frequency band of 30 to 60 MHz can be constructed. Moreover, since it has a configuration in which two interdigital transducers are connected in series, in other words, the piezoelectric substrate 2
Since this corresponds to a structure in which the interdigital transducer formed on 3 is divided into two, the capacitance value is reduced so that the capacitance value is sufficient for use in the AFT circuit. Moreover, since the surface acoustic wave resonator 20 is used, it is possible to omit the complicated work of adjusting the inductance L, which is required in the conventional LC resonant circuit.

Returning to FIG. 1, in this embodiment, the resonance circuit 19
Has a structure in which a resistor 21 and a capacitor 22 are connected in parallel to the surface wave resonator 20 using the BGS wave described above. The resistor 21 is a damping resistor for adjusting the AFT sensitivity, and is appropriately selected within the range of 100Ω to 100kΩ. The capacitor 22 is connected to cut the direct current component. In the resonance circuit 19 of the present embodiment, it is possible to give the AFT detector 8 a predetermined frequency discrimination characteristic without performing frequency adjustment as described above. Therefore, the AFT circuit does not need a structure for frequency adjustment, which contributes to downsizing of the AFT circuit.

In the above embodiment, an interdigital transducer in which two interdigital transducers are connected in series as shown in FIGS. 4 and 5 is formed on a piezoelectric substrate, but three interdigital transducers are used. The above interdigital transducers connected in series,
That is, in other words, a surface acoustic wave resonator having a structure in which one interdigital transducer is divided into three or more may be used. For example, in the surface acoustic wave resonator 30 schematically shown in FIG. 6, first to fourth comb-teeth electrodes 31 to 34 are formed on the piezoelectric substrate 33. That is, the first
The comb electrode 31 and the second comb electrode 32 form a first interdigital transducer, and the second comb electrode 32 and the third comb electrode 33 form a second interdigital transducer. Third comb electrode 33 and fourth
A third interdigital transducer is formed with the comb-teeth electrode 34, and the first to third interdigital transducers are connected in series.

[0017]

As described above, according to the present invention, the resonance circuit for giving a desired frequency discrimination characteristic in the AFT circuit of the television receiver is an end surface reflection type surface wave resonator utilizing BGS waves. And the resistance element connected in parallel to the surface wave resonator, it is possible to omit the complicated frequency adjustment work which is essential in the conventional AFT circuit. Moreover, since the structure for frequency adjustment is not required, the AFT circuit can be downsized. Therefore, it is possible to provide a small AFT circuit with excellent reliability.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a tuner portion of a television receiver including an AFT circuit according to an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram for explaining a tuner portion of a television receiver including a conventional AFT circuit.

FIG. 3 is a diagram showing operating characteristics of a frequency discriminating unit of an AFT circuit.

FIG. 4 is a perspective view showing a surface acoustic wave resonator using a BGS wave used in an embodiment of the present invention.

5 is a plan view for explaining an electrode shape of the surface acoustic wave resonator shown in FIG.

FIG. 6 is a perspective view showing another example of a surface acoustic wave resonator using BGS waves used in the present invention.

[Explanation of symbols]

 8 ... AFT detector 19 ... Resonance circuit 20 ... Surface wave resonator 21 ... Resistor

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuya Togawa 2-10-10 Tenjin, Nagaokakyo, Kyoto Prefecture Murata Manufacturing Co., Ltd.

Claims (1)

[Claims] 1. An AFT circuit that detects a video carrier of a television receiver and applies a detection output voltage corresponding to a deviation from a reference frequency to a local oscillation circuit, wherein a circuit that provides frequency discrimination characteristics is provided on a piezoelectric substrate.
A plurality of interdigital transducers composed of a pair of comb-shaped electrodes having a plurality of electrode fingers which are interleaved with each other are connected in series, and an end surface reflection type surface wave resonator utilizing a BGS wave and the surface wave are provided. An AFT comprising a resonator and a resistance element connected in parallel.
circuit.